Vertical gear-effect relies on the principle that impacts above or below the point of central impact (the “sweet spot”) on the face of a golf club cause the club head to rotate about its pitch axis (i.e., the heel-toe axis through the club-head center of mass) and, since the ball is in contact with a rotating striking surface, the ball also rotates but in the reverse direction. The spin directions of the club head and ball are likened to those in a pair of gear wheels.
The amount of imparted spin on the golf ball is found to be directly proportional to the distance of the club-head center of mass behind the impact face so golf clubs, such as irons, exhibit negligible gear-effect since each has its center of mass on, or close to, the impact face. By contrast, putters and fairway woods are commonly designed to have their center of mass some distance behind the impact face and can thus exhibit significant gear-effect.
In putters, vertical gear-effect is used to reduce or reverse imparted backspin. A ball launched on a putting surface with backspin loses more kinetic energy and pace through initial skidding compared to a ball with no backspin, or more preferably with overspin. This reduction of initial skid promotes ball roll and improves distance and (allegedly) direction control.
In fairway-woods, vertical gear-effect is used to increase ball carry by increasing elevation trajectory angle and reducing backspin. Most golf clubs are lofted and thus impart backspin to a golf ball by means of oblique impact. For distance shots, this backspin is a major advantage, since backspin gives the ball aerodynamic lift and allows it to remain airborne longer and thus fly longer. However, too much backspin increases aerodynamic drag (which reduces carry distance) and lifts the ball too much, so the ball climbs high in the air but at the expense of losing more distance. Vertical gear-effect can reduce this problem by contributing higher initial launch trajectory (as in high-lofted clubs) but counteracts the oblique-impact spin mechanism and reduces backspin.
An important requisite of gear-effect is that the golf club head behaves (at least to some extent) as a free body during impact. This “free body” behavior is established teaching in golf science and assumes that during the very brief time of contact (circa half a millisecond), the shaft has negligible influence on the outcome of the impact (see for example: A. Cochran and J. Stobbs, Search for the Perfect Swing, Chicago: Triumph Books, 1968, p. 147).
Thus, the launch velocities and spin vectors of a ball immediately after impact from a club head are predicted from a “free body model” of the ball and club head that ignores any effect of the mass or rigidity of the shaft. United States Patent Application Publication 2003/0013547 (Helmstetter et. al.) exemplifies such teaching of club-on-ball impact, where shaft effects are ignored and only the mass and inertial parameters of a club head, measured to several significant digits, are used to compute very small, theoretical differences in ball flight behavior. Any off-center impact on the club-face imparts rotation on the club head and the free body model teaches that this rotation occurs about an axis through the center of mass of the club head.